Intense Atomic and Molecular Beams Via Neon Buffer-gas
Cooling
The Harvard community has made this article openly available.
Please share how this access benefits you. Your story matters.
Citation
Patterson, David, Julia Rasmussen, and John M. Doyle. 2009.
Intense atomic and molecular beams via neon buffer-gas cooling.
New Journal of Physics 11(5): 055018.
Published Version
doi:10.1088/1367-2630/11/5/055018
Accessed
June 15, 2017 8:33:09 PM EDT
Citable Link
http://nrs.harvard.edu/urn-3:HUL.InstRepos:8866869
Terms of Use
This article was downloaded from Harvard University's DASH
repository, and is made available under the terms and conditions
applicable to Other Posted Material, as set forth at
http://nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.termsof-use#LAA
(Article begins on next page)
New Journal of Physics
The open–access journal for physics
Intense atomic and molecular beams via neon
buffer-gas cooling
David Patterson, Julia Rasmussen and John M Doyle1
Physics Department, Harvard University, Cambridge, MA, USA
E-mail: [email protected]
New Journal of Physics 11 (2009) 055018 (12pp)
Received 1 December 2008
Published 14 May 2009
Online at http://www.njp.org/
doi:10.1088/1367-2630/11/5/055018
We realize a continuous, intense, cold molecular and atomic beam
source based on buffer-gas cooling. Hot vapor (up to 600 K) from an oven
is mixed with cold (15 K) neon buffer gas, and then emitted into a high-flux
beam. The novel use of cold neon as a buffer gas produces a forward velocity
distribution and low-energy tail that is comparable to much colder helium-based
sources. We expect this source to be trivially generalizable to a very wide range
of atomic and molecular species with significant vapor pressure below 1000 K.
The source has properties that make it a good starting point for laser cooling
of molecules or atoms, cold collision studies, trapping, or nonlinear optics in
buffer-gas-cooled atomic or molecular gases. A continuous guided beam of cold
deuterated ammonia with a flux of 3 × 1011 ND3 molecules s−1 and a continuous
free-space beam of cold potassium with a flux of 1 × 1016 K atoms s−1 are
realized.
Abstract.
1
Author to whom any correspondence should be addressed.
New Journal of Physics 11 (2009) 055018
1367-2630/09/055018+12$30.00
© IOP Publishing Ltd and Deutsche Physikalische Gesellschaft
2
Contents
1. Introduction
1.1. Buffer-gas cooling and direct injection of atoms and molecules
2. Apparatus
3. Thermalization and loss to the cell walls
4. Neon as a buffer gas
5. Potassium results
6. Applications
6.1. Demonstrated: loading an electrostatic beamguide . . . . . . .
6.2. Next step I: collisional studies . . . . . . . . . . . . . . . . .
6.3. Next step II: general loading of traps . . . . . . . . . . . . . .
6.4. Next step III: nonlinear optics in buffer-gas cells . . . . . . . .
7. Conclusion
Acknowledgments
References
. . . . . . . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
2
2
3
4
5
6
7
7
9
10
10
11
11
11
1. Introduction
Cold, slow, laser-cooled atomic sources of atoms have been laboratory standards for over a
decade but only recently have cold, slow sources of molecules been demonstrated [1]. This is
largely because the complex level structure of molecules makes laser cooling, the workhorse of
atom cooling, very difficult for the vast majority of molecules. (Laser cooling of molecules has
yet to be demonstrated, although a recent proposal indicates a path towards this with some
molecules [2].) Techniques to produce cold, low-velocity samples of molecules are buffergas cooling [3], slow molecule filtering from a warm, effusive source [4]–[6], mechanical
slowing [7], deceleration of molecules from a seeded supersonic source using time-varying
electric fields [1, 8, 9], photoassociation of cold and ultracold atoms [10], and the formation of
molecules from ultracold atoms using Feshbach resonances [11]. In general, the fluxes achieved
with these molecular sources have been far lower than those achieved in cold atom sources.
1.1. Buffer-gas cooling and direct injection of atoms and molecules
Cryogenic helium buffer-gas cooling has proven to be a versatile tool for producing cold, dense
stationary gases and high-flux beams of cold atoms and molecules [3, 12]. The species of interest
is injected into a cell containing cold helium gas where it thermalizes, producing a cold mixture
with the helium. Until recently, the primary methods of injection have been laser ablation, in
situ electric discharge and direct introduction of hot molecular beams. Ablation and discharges
deposit far more energy into the buffer gas than is necessary for cooling the injected species. For
example, in laser ablation of a typical metal, a laser pulse with an energy of ≈10 mJ will result
in ≈1012 buffer-gas-cooled atoms. Assuming a heat of vaporization of 1000 K per atom, this
represents an efficiency of 10−4 [13]. Even in the best cases, low efficiencies limit the available
flux of cold atoms or molecules, and generally requires experiments to be run in a pulsed mode.
New Journal of Physics 11 (2009) 055018 (http://www.njp.org/)
3
Intermediate region
100 K
Needle 550 K
Cell
15 K
Adjustable
aperture,
1-5 mm dia.
Thermal standoffs
Potassium oven
550K
Liquid potassium
300 K
20 K
Detection lasers
Neon buffer gas inputs
Figure 1. Overview of apparatus. An oven supplies a hot (550 K) mixture of
potassium and neon that flows through an intermediate region (100 K) to a
cold tube ‘cell’ (15 K). The oven, intermediate plate, and cells are separated by
thermal standoffs to reduce heat loads. A cold beam of potassium and neon exits
an adjustable aperture on the far side of the cell where its characteristics are
measured by absorption spectroscopy.
Directly injecting a hot molecular beam is very energy efficient, but so far has been limited to
peak input fluxes of 5 × 1013 molecules s−1 in 10 ms pulses [14].
Using helium buffer-gas cooling, we previously [12] produced a continuous beam of
cold molecular oxygen, 16 O2 guided 30 cm from a cryogenic cell to a room temperature
UHV chamber. In that work, oxygen gas was flowed into the cold (T ≈1.5 K) helium buffer
gas through a T ≈ 60 K capillary line. The oxygen cooled and then exited through a 4 mm
diameter aperture. The resultant beam was used to load a magnetic guide at high molecular
flux. In this work, we present substantial developments of this technique and demonstrate them
by continuous production of large fluxes of cold potassium atoms and deuterated ammonia
molecules. We employ a much hotter capillary (T = 600 K) and use, in addition to helium, a
neon buffer gas. We show that the use of neon as a buffer gas leads to beams with more slow
atoms or molecules than helium-based beams, even though the neon gas is at a significantly
higher temperature (due to vapor pressure constraints). This new variant of buffer-gas beam
sources represents a major departure from previous approaches and can be trivially generalized
to provide a continuous, high-flux source of essentially any species that has a significant vapor
pressure below 1000 K.
2. Apparatus
Our potassium beam apparatus is centered around a cold tube (the ‘cell’), T = 6–15 K, into
which we flow target atoms or molecules and inert gas, as depicted in figure 1. This cell is
the coldest part of the apparatus and is 1 cm in diameter and 8 cm long. A hot (up to 600 K)
gas mixture of neon and potassium flows into one end of the tube. The mixture cools to the
temperature of the tube walls as it flows down the tube and out into vacuum, realizing the beam.
In more detail, a hot (up to 600 K) copper tube is attached via a 2 cm long stainless steel
tube to a copper plate (about 100 K), which in turn is anchored to the first stage (40 K) of
New Journal of Physics 11 (2009) 055018 (http://www.njp.org/)
4
a closed cycle pulse-tube refrigerator [15]. This plate absorbs the bulk of the radiative and
conductive heat load from the hot input tube, as well as possibly precooling the gas mixture. The
tube continues via a 1 cm long, 1 cm diameter, thermally isolating section of epoxy/fiberglass
composite and enters the cold cell through a 6 mm aperture in a copper plate. The cold cell is
thermally connected to the second stage (6 K) of the pulse tube refrigerator and has an adjustable
aperture (typically 3 mm diameter) at the far end. Total heat loads on the cryorefrigerator from
the apparatus are about 40 W at the first stage and 1 W at the second stage, which coincides both
with the cooling power of the pulse tube and with typical heat loads in a nitrogen/helium liquid
cryogen cryostat. The total distance between the end of the hot copper tube and the entrance
to the cell is 1.5 cm. Our current maximum running temperature of 600 K is limited by this
40 W heat load on the first stage of the cryocooler, and could be significantly increased by better
thermal isolation, or the construction of an additional intermediate cooling plate at 300 K.
Buffer gas can be introduced—in combination or separately—at the oven (600 K), the first
copper plate (100 K) or the cell (15 K). The temperature of the gas mixture at the exit of the
cell is measured to be ≈15 K regardless of where the gas is introduced (see section 3 below);
the flux of cold potassium is maximized when the buffer gas flow through the oven dominates
over the flow into the plate or cell, likely because this flow transports the comparatively lowpressure potassium (1 Torr at 600 K) down the long hot copper tube and cooling occurs directly
within the cell. Our potassium flux is limited by the vapor pressure of potassium at our current
maximum running temperature of 600 K, suggesting that fluxes could be substantially higher if
we were using a species with a higher vapor pressure. The oven and transport tube could be run
at higher temperature with straightforward engineering improvements to the thermal isolation.
Further details of the apparatus, including the design for our high-speed charcoal cryopumps,
are described in [12].
3. Thermalization and loss to the cell walls
The key physical process that leads to production of cold potassium is the flow and
thermalization of the initially hot potassium/buffer-gas mixture within the cell. In order to
realize a cold, high-flux beam, the experimental conditions generally need to satisfy two
competing conditions. The gas mixture must thermalize with the cold walls, and the mixture
must exit the cell (a tube) before much of the potassium diffuses to the walls, where it has a
high probability of permanently sticking.
It is not clear a priori that it is possible to achieve thermalization without significant atom
loss. Assuming a tube of radius r , a neon density of n Ne , a gas temperature T , and hard sphere
elastic cross sections of σNe–Ne and σK–Ne , respectively, the characteristic time τ for the gas to
thermalize with the walls is
τ = r 2 n Ne σNe–Ne (m Ne /2kB T )1/2 .
Assuming that n Ne n K , the diffusive loss of potassium to the walls will occur on a
timescale τloss :
τloss = r 2 n Ne σK–Ne (m Ne /2kB T )1/2 .
These expressions imply that in order to thermalize with only order unity loss in atom
flux, we need σNe–Ne . σK–Ne . Although we did not make accurate density measurements in
New Journal of Physics 11 (2009) 055018 (http://www.njp.org/)
5
our cell under actual running conditions due to the high (>100) optical density, we have
evidence that loss to the cell walls is minimal; we have been unable to find a regime where the
temperature of the beam differs significantly from the measured temperature of the cell walls.
We therefore speculate that σNe–Ne < σK–Ne , but note that this conclusion and the sensitivity of
the overall cooling process to the ratio of these cross sections remains open to both theoretical
and experimental investigation.
4. Neon as a buffer gas
In order to achieve the lowest possible temperature, the overwhelming majority of buffer-gas
cooling experiments to date have used helium as the buffer gas. With a sufficiently high vapor
pressure at temperatures as low as 300 mK (for 3 He), compared to a lower limit of ≈12 K for
neon, helium is the natural choice when low temperature is the only figure of merit.
Both magnetic and electrostatic traps have depths of at most a few K. It is therefore natural
to maximize the flux of molecules with a total energy less than a given energy E trappable ≈1 K,
assumed here to be less than Tcell ≈4–20 K. Somewhat counterintuitively, by this metric a
hydrodynamic [12] beam using 15 K neon as the buffer gas can substantially outperform a
similar beam using 4 K helium.
Consider a beam consisting of a small fraction of potassium ‘impurities’ of mass m K mixed
with a much larger flow of a noble gas of mass m. The mixture is assumed to thermalize to a
temperature T before it exits the nozzle. Depending on the number of collisions within and just
outside the nozzle, such a beam can operate in one of three regimes:
1. Effusive, characterized by N < 1 collision. In such beams, only the atoms in the cell which
happen to diffuse through the small nozzle aperture are emitted into the beam. These beams
have a forward velocity of order v̄ ≈ (2kB T /m K )1/2 and relatively low flux.
2. Hydrodynamic, characterized by 1 < N . 100 collisions; the flow through the nozzle
is fluid dynamic, but isothermal. These beams typically have a large flux enhancement
compared to effusive beams due to hydrodynamics within the cell, as described
in [12]. Hydrodynamic beams have a modestly boosted mean forward velocity of v̄ ≈
(2kB T /m)1/2 .
3. Fully supersonic, characterized by N > 1000 collisions in and outside the nozzle. In this
regime, there is significant adiabatic cooling as the gas expands into the vacuum; the
forward velocity distribution is sharply peaked around a velocity v̄ & 2 × (2kB T /m)1/2 .
Figure 2 shows the forward energy spectrum to be expected in each of these regimes.
In order to take advantage of large flux enhancements, we run our beams in the
hydrodynamic regime described above (blue curve in figure 2). Figure 3 shows the simulated
forward velocity distribution for such a source using 4.2 K helium, 15 K neon and 40 K argon as
the buffer-gas. It is clear that despite the higher temperature required for a neon or argon beam,
there are more cold, low-energy atoms in a neon-cooled beam than in a helium-cooled beam.
Using neon instead of helium as the buffer gas carries several important technical
advantages as well. Most notably, neon has a negligible vapor pressure at our cryostat’s base
temperature of 5 K, meaning that any cold surface becomes a high-speed cryopump. The
vacuum outside the cell can be maintained at a high level (1 × 10−6 Torr in the main chamber
and <10−8 Torr in a differentially cryopumped ‘experiment chamber’) even with a high flow of
New Journal of Physics 11 (2009) 055018 (http://www.njp.org/)
6
0.45
0.4
Hydrodynamic
0.35
Supersonic
Relative density
0.3
0.25
0.2
0.15
0.1
Effusive
0.05
0
0
50
100
150
200
250
300
Beam forward velocity (m s−1)
Figure 2. Simulated forward velocity distributions for buffer gas beams in
various regimes. As a mixture of potassium and neon buffer gas emerges from
the nozzle, a typical potassium atom collides with neon atoms as it moves away
from the nozzle aperture. Qualitatively, effusive beams suffer zero collisions,
‘hydrodynamic’ beams a few collisions, and supersonic beams many collisions,
remaining fluid dynamic for many aperture diameters. The beams demonstrated
in this paper are hydrodynamic, but generally not supersonic (blue curve).
order 1019 neon atoms s−1 through the cell. In future experiments, simple shutters could be used
to rapidly close off this differentially pumped chamber, which we believe will rapidly achieve
an extremely high vacuum. The higher temperature means the cell can tolerate substantially
higher heat loads. The principle disadvantage of neon is that the higher temperature leads to
less rotational cooling in molecules than that available from 4 K helium.
5. Potassium results
A cold, high-flux beam of potassium was characterized to study this new beam system. The
density, transverse velocity distribution and longitudinal velocity distribution are inferred from
the amplitude and Doppler broadening of the absorbtion spectra. Two lasers (potassium D2
line, 767 nm) are used to characterize the atomic beam. The first, used to measure the beam flux
and transverse velocity distribution, passes through the center of the combined neon/potassium
beam 2 cm away from the nozzle.
The second laser propagates parallel to and slightly below the atomic beam, intersecting
the edge of the beam. This laser is used to measure the forward velocity distribution of the beam.
Figure 4 shows typical measured velocity distributions.
Under typical running conditions (potassium oven and injection needle heated to 580 K,
the cell held at 17 K, 110 sccm of neon buffer-gas flowing), we measure an optical density of
about five in the transverse laser path, and a mean forward velocity of 130 m s−1 , in excellent
agreement with a simple simulation of a hydrodynamic beam at 17 K. These measurements
New Journal of Physics 11 (2009) 055018 (http://www.njp.org/)
7
0.4
0.35
4.2 K helium
Relative density
0.3
0.25
40 K argon
0.2
14 K neon
0.15
0.1
0.05
0
0
50
100
150
200
250
300
Beam forward velocity (m s−1)
Figure 3. Theoretical energy spectrum for hydrodynamic beams using 4.2 K
helium, 14 K neon, or 40 K argon as a buffer gas. The portion of the distribution
with an energy of less than 4 K is shown in bold. Neon is clearly an attractive
choice for experiments interested in maximizing the flux of very cold atoms or
molecules.
imply a density of about 8 × 1011 potassium cm−3 with an atomic beam diameter of about 1 cm
and a total beam flux of 1 × 1016 atoms s−1 . The beam divergence is measured to be about 0.7 sr,
consistent with our model of beams in this hydrodynamic regime.
6. Applications
6.1. Demonstrated: loading an electrostatic beamguide
It is natural for a variety of experiments to want a cold beam of molecules that has been
separated from the helium or neon buffer gas. As an example application of this source, we
have demonstrated loading a cold beam of ND3 into a curved electrostatic guide; the output
of such a guide consists of pure, state-selected ND3 molecules. (We note that after our ND3
experimental work was completed we learned of similar results presented in [16].) In addition,
the guide acts as a filter that passes only molecules moving slowly enough to be guided around
the bend. Such a guide can be used to bring the beam into a differentially pumped region, either
cryogenic (this work) or at room temperature [12, 16].
The apparatus for our guided ND3 work is simpler than for our high-flux potassium
beam. Specifically, the ND3 is fed directly into the cell and the capillary temperature is a
lower T = 280 K. Cold ND3 molecules exit the cell and after a short gap of 30 mm, enter an
electrostatic guide. ND3 molecules exiting the guide were detected via 2 + 1 REMPI using a
6 mJ pulsed laser at 317 nm. The guide consists of six 1 mm diameter stainless steel rods with a
center–center spacing of 2 mm. The guide entrance is positioned 30 mm from the exit aperture
of the cell, slightly beyond the expected position of the last collision between a typical ND3
molecule and buffer gas atom. The position of this last collision is velocity dependent, with slow
New Journal of Physics 11 (2009) 055018 (http://www.njp.org/)
8
(a) Transverse velocity distribution
Optical density
1
Data
17 K theory
0.5
0
−200
−150
−100
−50
0
50
100
150
200
Velocity (m s−1)
Relative absorbtion signal
(b) Forward velocity distribution
1
Data
17 K theory
0.5
0
0
50
100
150
200
250
300
Velocity (m s−1)
Figure 4. Measured transverse (a) and forward (b) velocity distribution of a
high-flux beam of potassium. The green curves represent the Doppler broadened
spectrum expected from the velocity distribution of a hydrodynamic beam with
Tgas = 17 K. The amplitude is the only free parameter in these fits. The left side
of each plot is somewhat distorted by the nearby F = 0 hyperfine line. These data
represent a beam with a potassium flux of 1 × 1015 atoms s−1 ; we have observed
fluxes as high as 1 × 1016 atoms s−1 , but they are not shown here because the high
optical density (≈8) makes their interpretation less straightforward.
molecules suffering more collisions. We therefore speculate that the dominant loss mechanism
for the very slowest molecules in the guided velocity distribution is collisions with buffer-gas
atoms in the first few cm of the guide. A schematic of the guide and ion detection assembly is
shown in figure 5.
The guide is constructed from alternating rods held at 5 kV and ground, producing a linear
hexapole field. The guide rods were held in place with Ultem brackets; the entire assembly
was mounted to the 6 K cold plate. No detectable (<0.1 µA) leakage current flowed along the
surface of the Ultem brackets despite the fact that few precautions were taken to avoid such
currents. We speculate that such surface currents are suppressed at low temperatures. The guide
is 70 cm long and describes a complete loop with a bend radius of 8 cm. The first 30 cm of the
guide can be switched on or off independently from the rest of the guide.
In a typical experimental run, the first 30 cm of the guide (the ‘gate’) is initially turned
off and the remainder of the guide is charged. The first section is then turned on, and, after a
variable delay, the pulsed REMPI laser is fired, ionizing ammonia molecules a few cm away
from the guide exit. These ions are then detected by a multi-channel plate.
The velocity distribution in the guide, shown in figure 6, is inferred by varying the delay
between opening the gate and detection. We can only present a conservative lower bound on
the guided molecule flux as absolute density measurements are notoriously unreliable in MPI
New Journal of Physics 11 (2009) 055018 (http://www.njp.org/)
9
Electrostatic ‘gate’
E = 100 V cm–1
MCP
Cell
Electrostatic guide
Pulsed laser
(317 nm, ~4 mJ)
ionizes molecules
Slow, guided ND3
at guide output
Figure 5. Apparatus overview for guiding and detecting ND3 molecules.
As in [4], the bent guide passes only slow moving low field seeking molecules.
experiments; our quoted flux is based on very conservative comparisons with similar, calibrated
experiments on ND3 done in other groups [17]. With our most sharply bent guide (figure 6(b)),
we find a lower bound of 3 × 108 guided molecules s−1 with an average energy corresponding
to a temperature of about 4 K.
The broad linewidth (≈5 GHz) of the pulsed laser used for the 2 + 1 REMPI spectroscopy
makes direct measurement of the beam velocity via Doppler spectroscopy impossible. The
longitudinal velocity distribution is measured via the gate/time-of-flight method described
above, while the transverse velocity distribution is limited by the guide depth of ≈600 mK.
In addition, the rotational temperature of both the guided and unguided beams was found to
be less than 20 K in all cases, and in excellent agreement with rotational ND3 spectra taken in
supersonic beams [17].
The ND3 beam was initially demonstrated using both helium and neon as a buffer gas.
Figure 6(a) shows the measured velocity distribution of a guided beam of ND3 using helium
and neon as buffer gas under otherwise similar conditions. The guide used in this work has
a significantly larger (70 cm) radius of curvature, leading to a much higher velocity cutoff
in the beam guide output velocity distribution. Significantly more slow molecules are produced
in the beam using neon buffer gas.
6.2. Next step I: collisional studies
There is substantial interest in studying both elastic and inelastic collisions of molecules at low
energies [1]. The guided output of the deuterated ammonia beam described above represents a
pure, rotationally cold, state selected, slow molecular beam. In addition, a particular velocity
class can be selected by controlled switching of the guide. Such a beam could be crossed with a
similar beam, a laser cooled atomic beam, a trapped sample, or a cold thermal beam of hydrogen,
helium, or neon, providing a ‘lab bench’ to study cold collisions with energies of a few cm−1 .
Such a system would provide an attractive set of ‘knobs’ to the experimenter—collision energy,
incoming rotational state and external fields can all be controlled. Even a small fraction of the
beam being scattered inelastically into high-field seeking states would be detectable, since the
output of the beam guide consists of very pure low-field seeking states.
New Journal of Physics 11 (2009) 055018 (http://www.njp.org/)
10
(a)
1.2
(b)
0.03
1
12 K neon buffer gas
12 K helium buffer gas
0.025
0.02
Relative ion signal
Relative ion signal
0.8
0.6
0.4
0.015
0.01
0.2
0.005
0
0
−0.2
−0.005
0
50
100
150
–1
Beam forward velocity (m s )
200
0
20
40
60
80
100
–1
Beam forward velocity (m s )
Figure 6. (a) Measured forward velocity distribution of a guided ND3 beam
using 12 K helium (blue square) and 12 K neon (red triangle) as a buffer gas. The
beam using neon contains substantially more low-velocity molecules. Molecules
moving faster than ≈150 m s−1 are unlikely to be guided by this bent electrostatic
guide (bend radius r = 70 cm), although the exact fraction guided depends on the
transverse energy of the molecules. The red data (neon) represents a guided beam
of about 2 × 1010 ND3 molecules s−1 at a mean energy of ≈20 K. A background
count rate has been subtracted. (b) Velocity distribution at the output of a
70 cm long electrostatic guide with a bend radius of 8 cm. The output of this
guide is completely separated from the neon buffer gas, and represents a pure,
state-selected, low energy beam. This data represents 3 × 108 ND3 molecules s−1 ,
with a mean energy of about 4 K.
6.3. Next step II: general loading of traps
Low velocities and high fluxes make this source an attractive first step for loading electrostatic or
magnetic traps. In order to load atoms or molecules into a conservative potential, energy must be
removed from the particles while they are within the trap volume. Demonstrated methods to load
traps from beams include using light forces [18, 19], switching electric [17] or magnetic [20]
fields and buffer-gas loading [14]. Chandler et al [21] have proposed loading traps via collisions
with another beam, and it should also be possible to optically pump slow moving atoms or
molecules directly into trapped states [22]. Recent proposals [2] suggest molecules as well as
atoms could be laser cooled from a source like ours, and all other methods listed here are trivially
generalizable to molecules. All of these methods would benefit from starting with our bright,
cold, slow moving source.
6.4. Next step III: nonlinear optics in buffer-gas cells
There has been recent interest [23, 24] in studying nonlinear opticals effects in cold buffer-gas
cells. There is theoretical [23] and experimental [25] evidence that decoherence times between
ground-state magnetic sublevels in alkali atoms are very long. To date, only cells loaded via
laser ablation, and in one case light-induced desorbtion [26], have been used for this work.
Although substantial optical densities have been achieved, such experiments tend to be plagued
New Journal of Physics 11 (2009) 055018 (http://www.njp.org/)
11
by anomalous losses at high buffer-gas pressures, limiting lifetimes to <100 ms. Ablation is
a violent and difficult to control process which is known to produce both clusters [27] and
momentary dramatic heating [13] in buffer-gas cells. We believe that continuously loading
cold vapor cells directly from an oven will not only provide higher density samples, but also
do so controllably in a clean environment. Possible demonstrations include improved EIT [24],
guiding light [28] and high-resolution magnetometry [29]. We have already achieved in the
experiments described above continuous in-cell potassium optical densities much greater
than 100.
7. Conclusion
We have demonstrated an extremely high-flux and moderately cold atomic beam of potassium
using a neon buffer gas. We believe this source can be trivially generalized to any atomic and
many molecular species that can be produced in a gas at temperatures up to 600 K and perhaps as
high as 1000 K. The demonstrated cold flux of potassium represents a new benchmark for cold
atom fluxes. In addition, an ND3 from such a source has been coupled into an electrostatic beam
guide. A broad range of possible applications with such guided molecules includes collision
studies, trap loading and nonlinear optics studies.
Acknowledgments
This work is supported by the National Science Foundation (Grant 0551153).
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
Bethlem H L and Meijer G 2003 Int. Rev. Phys. Chem. 22 73
Stuhl B K, Sawyer B C, Wang D and Ye J 2008 Phys. Rev. Lett. 101 243002
Weinstein J D, deCarvalho R, Guillet T, Friedrich B and Doyle J M 1998 Nature 395 148
Rangwala S A et al 2003 Phys. Rev. A 67 043406
Nikitin E, Dashevskaya E, Alnis J, Auzinsh M, Abraham E R I, Furneaux B R, Keil M, McRaven C,
Shafer-Ray N and Waskowsky R 2003 Phys. Rev. A 68 023403
Deachapunya S, Fagan P J, Major A G, Reiger E, Ritsch H, Stefanov A, Ulbricht H and Arndt M 2008
Eur. Phys. J. D 46 307
Gupta M and Herschbach D 2001 J. Phys. Chem. A 105 1626
Boxhinski J, Hudson E R, Lewandowski H, Meijer G and Ye J 2003 Phys. Rev. Lett. 91 243001
Tarbutt M R, Bethlem H L, Hudson J J, Ryabov V L, Ryzhov V A, Sauer B E, Meijer G and Hinds E A 2004
Phys. Rev. Lett. 92 173002
Sage J M, Sainis S, Bergeman T and DeMille D 2005 Phys. Rev. Lett. 94 203001
Kokkelmans S, Vissers H and Verhaar B 2001 Phys. Rev. A 63 031601
Patterson D and Doyle J 2007 J. Chem. Phys. 126 154307
Michniak R 2004 PhD Thesis Harvard University
Campbell W, Tsikata E, Lu H-I, van Buuren L and Doyle J 2007 Phys. Rev. Lett. 98 213001
Doyle J 2008 Model PT410 (Cryomech Inc.)
van Buuren L, Sommer C, Motsch M, Pohle S, Schenk M, Bayerl J, Pinkse P and Rempe G 2008
arXiv:0806.2523v1
Bethlem H L, Berden G, Crompvoets F M H, Jongma R T, van Roij A J A and Meijer G 2000 Nature 406 491
Chu S, Cohen-Tannoudji C and Phillips W D 1998 Rev. Mod. Phys. 70 685–741
New Journal of Physics 11 (2009) 055018 (http://www.njp.org/)
12
[19] Helmerson K, Martin A and Pritchard D 1992 J. Opt. Soc. Am. B 9 1988–96
[20] Narevicius E, Parthey C G, Libson A, Narevicius J, Chavez I, Even U and Raizen M G 2007 New J. Phys. 9
358
[21] Elioff M S, Valentini J and Chandler D W 2003 Science 302 1940
[22] Stuhler J, Schmidt P O, Hensler S, Werner J, Mlynek J and Pfau T 2001 Phys. Rev. A 64 031405
[23] Sushkov A O and Budker D 2008 Phys. Rev. A 77 042707
[24] Hong T, Gorshkov A V, Patterson D, Zibrov A S, Doyle J M, Lukin M D and Prentiss M G 2009 Phys. Rev.
A 79 013806
[25] Hatakeyama A, Oe K, Ota K, Hara S, Arai J, Yabuzaki T and Young A R 2005 Phys. Rev. Lett. 84 1407
[26] Hatakeyama A, Enomoto K, Sugimoto N and Yabuzaki T 2002 Phys. Rev. A 65 022904
[27] Hachimi A and Muller J 1997 Chem. Phys. Lett. 268 485
[28] Vengalattore M and Prentiss M 2005 Phys. Rev. Lett. 95 243601
[29] Budker D, Gawlik W, Kimball D, Rochester S, Yashchuk V and Weis A 2002 Rev. Mod. Phys. 74 1153
New Journal of Physics 11 (2009) 055018 (http://www.njp.org/)